os/exec: add more caveats to StdoutPipe, StderrPipe
os/exec: add more caveats to StdoutPipe, StderrPipe

(StdinPipe was taken care of by CL 13329043.)

Fixes issue 6008.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13606046

pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go